Felodipine and nitrendipine, 1 and 2, represent effective medicines useful for the treatment of hypertension and as muscle relaxant drugs. They belong to a class of medicines collectively known as dihydropyridines. 
The preparation of felodipine (ethyl methyl 4-(2,3-dichlorophenyl)-1,4-dihydro-2,6-dimethyl-3,5-pyridine dicarboxylate) and related compounds typically involves a multistep protocol as depicted in Scheme 1. 
The acidic- and base-catalyzed condensation of benzaldehyde with an alkyl acetoacetate to form a key benzylidene intermediate of formula 4 is known. However, it is also well understood in the art that the performance of this condensation is very sensitive to the chemical nature of both base and acid. In particular, when basic catalysis is used, the benzylidene can be formed at a low temperature, but then reacts further with another molecule of the alkyl acetoacetate to form the bis-adduct (6, Scheme 2) as an impurity. This disadvantage is further compounded for the intermediate used in the preparation of felodipine since the requisite benzaldehyde component is 2,3-dichlorobenzaldehyde. When the resulting dihydropyridine molecule is made in the presence of these impurities, the chlorine atoms on the aromatic ring make the carbon atom of the aldehyde more electron deficient relative to benzaldehyde, further favouring the formation of the undesired bis-adduct 6. When acid is used as catalyst for this condensation, for instance as described in U.S. Pat. No. 5,310,917, a mixture of aldol by-products can also be formed. 
The use of piperidinium acetate as catalyst by Arthur C. Cope (Journal of the American Chemical Society, 1937, 59, 2327-2330) represents a landmark for making benzylidene compounds. For example, U.S. Pat. No. 4,600,778 disclosed a process for making nitrendipine and felodipine using this catalytic system in an alcoholic solvent. Novel catalytic systems, which purportedly overcome some of the deficiencies in the synthetic procedures disclosed in U.S. Pat. No. 4,600,778, are described in U.S. Pat. No. 5,977,369. However, although the benzylidene intermediate could be obtained as a mixture composed of two isomers, the yield still was only about 60% thereby making it undesirable for commercial production. Summarizing, the major disadvantages with the disclosed processes for the preparation of dihydropyridine compounds, particularly felodipine, from benzylidene intermediate processes of the prior art, include:                1. Formation of symmetrical diesters (dimethyl and diethyl, 7 and 8, respectively, Scheme 2) byproducts, which are very difficult to remove from the product.        2. Extractive workup for isolation of the desired product.        3. Purification by recrystallization requiring increased manufacturing time and solvent costs.        4. Low overall yield.        
Therefore, a catalytic system combining an optimal balance of base and acid properties, which would provide the benzylidine intermediates in high yield and with a minimum number of side-products, was required.
It is accordingly an objective of the present invention to provide such a novel, simple and efficient process for the manufacture of benzylidene intermediates useful in the preparation of dihydropyridine compounds and, such as the dihydropyridine molecule, felodipine that overcomes the deficiencies in the prior art.